The present invention relates, in general, to the field of frequency shift keying detector circuits and methods utilized for the demodulation of a digital data signal from a modulated sine wave carrier signal. More particularly, the present invention relates to a simple, low parts count frequency shift keying detector and method which is readily implemented in an integrated circuit for the detection of a digital data signal in wireless applications without the use of an internal reference frequency.
In its simplest terms, a digital data signal may be used to modulate the amplitude, the frequency or the phase of a sine wave carrier depending upon the particular application. Respectively, these three types of modulation are known as amplitude shift keying ("ASK") frequency shift keying ("FSK") and phase shift keying ("PSK"). In any of these modulation techniques, the modulated carrier takes on one of two states, that is, either one of two amplitudes, two frequencies, or two phases. The two states of the modulated signal then represent either a logic "zero" or a logic "one".
As noted above, changing the frequency of the modulated signal to denote either of two digital logic states is called FSK modulation. FSK modulation offers a number of advantages in certain applications over other modulation techniques with respect to noise immunity and average signal power level. However, in conventional FSK detectors, it is necessary to have a phase locked loop ("PLL") or other frequency detection circuitry associated with the detector in order to determine whether a change in frequency denoting a data bit "one" or "zero" has occurred.
Therefore, while it would be advantageous to use FSK modulation techniques to transmit data to, for example, a passive radio frequency ("RF") identification ("ID") receiver or transponder, it has heretofore been impractical to incorporate an on-board PLL or other frequency detection circuit in such a device. Moreover, due to the packaging, and hence integrated circuit die size constraints imposed by such applications, the FSK detector must be effective, readily integratable and consume little on-chip "real estate".